Previous methods of grating production have been to rule (by either mechanical or holographic means) lines upon a preformed (ground and polished) substrate. This substrate is nearly always a section of a plane (flat), a section of a right circular cylinder, a section of a sphere, or a section of a toroid, depending on the purpose. For the case of reflection gratings, the rulings or lines placed on the substrate alternately absorb and reflect radiation incident upon the grating such that diffraction of the radiation takes place. For the case of transmission gratings, the substrate is either transparent to the radiation of interest or is removed (leaving a free-standing grid). Diffraction takes place in a manner similar to that of the reflection grating, but in this case the lines or rulings alternately absorb and transmit the radiation.
The disadvantages of the old method are a direct consequence of the limited choice of substrate shape. While a plane (or flat) grating (if suitably ruled) is ideal for diffraction of plane-wave radiation, the other grating shapes are only approximately suited to the diffraction geometries for which they are designed. Stigmatic images can now be produced at one (or at most three wavelengths) by producing the lines on the substrate by holographic means, and as long as the wavelengths of interest lie in close proximity to one of these three wavelengths, gratings produced by state-of-the-art means are usually adequate. But, for examination of spectra over a broad wavelength range, spectroscopists have had to be either content with wavelength resolutions of less than 1000, or have been forced to introduce other optical elements (mirrors and/or lenses) in the optical path in order to reduce the divergence of the incident beam. The disadvantage inherent in the latter case is that at ultraviolet (UV) and X-ray wavelengths every optical element introduced produces large attenuations of the incident radiation, sufficiently large in many cases to reduce the radiation to a point where it is immeasurable.
A diffraction grating for examination of spectra over a broad wavelength range in the UV and X-ray wavelengths is necessary to more efficiently analyze the large number of cosmic sources that are expected to emit line radiation in the soft X-ray and far ultraviolet region of the spectrum. Optically thin sources from which line spectra are expected include flare star and hot stellar coronae, supernova remnants, contact binaries, and perhaps accretion disks around compact objects. Absorption features may be observed in some stellar objects of astrophysical interest as well as in the interstellar medium. In addition, should the resolution be large enough, Doppler shifts and line broadening will also be observed.